Electric drive motors, in particular DC motors or brushless DC motors (EC motors), are known as electric drives for a tool of a handheld work apparatus. Such portable work apparatuses can be drills, rechargeable screwdrivers, motor-operated chainsaws, angle grinders, blowers, brushcutters, hedge trimmers or similar work apparatuses.
In order to operate a brushless electronically commutated three-phase DC motor (BEC motor or merely EC motor), an electronic control unit for an actuation circuit is necessary, by means of which the phase connections of the stator are energized in order to generate a rotating field driving the rotor of the DC motor. Such an actuation circuit is described in U.S. Pat. No. 8,493,011 and preferably comprises electronic power switches such as MOSFETs or the like. Switchover of the energization of the stator from first phase connections to next phase connections is performed depending on the position of the rotor, which needs to be established by suitable measures. Often, magnetic field sensors, so-called Hall sensors, are used for identifying the rotor position, which sensors respond to the magnetic field of the rotating rotor and output an output signal. If, for example, three Hall sensors are arranged over the circumference of the stator, the control unit can identify the rotary position of the rotor with sufficient precision by evaluation of the output signals of the Hall sensors and effect energization, matched to the rotary position, of the associated field coil.
Handheld, portable work apparatuses such as, for example, motor-operated chainsaws require a safety brake device, as is described, for example, in U.S. Pat. No. 8,100,234, for quickly bringing the tool, for example the saw chain, to a standstill. Such safety brake devices are arranged on the output side of the drive and are intended to ensure that the tool is brought to a standstill quickly. If the safety brake device is triggered, the tool is braked to a standstill in the range of approximately 120 msec.
If electric drive motors are used in handheld, portable work apparatuses, for example in motor-operated saws, electrical brake systems as documented in U.S. Pat. No. 4,680,862 are also used. The driving electric motor can be braked by short-circuiting its field coils, but the braking times which can be achieved in the process are unsatisfactory and cannot replace a safety brake device. Therefore, the safety brake devices in the form of band brakes are still necessary for achieving braking of the tool within the time range of approximately 120 msec.
If handheld, portable work apparatuses are in the form of rechargeable devices the work apparatus has a high total weight owing to the necessary rechargeable battery pack; the additional brake devices required for bringing the tool to a standstill quickly increase this weight further since the safety brake devices are complex in design terms and heavy.